Field of the Invention
The present disclosure relates to an organic light emitting diode display having high luminescence. Especially, the present disclosure relates to an organic light emitting diode display having high luminescence with low power consumption by enhancing the capacity of anode electrode and the capacitance of organic light emitting diode itself.
Discussion of the Related Art
Nowadays, various flat panel display devices are developed for overcoming many drawbacks of the cathode ray tube such as heavy weight and bulk volume. The flat panel display devices include the liquid crystal display device (or LCD), the field emission display (or FED), the plasma display panel (or PDP) and the electroluminescence device (or EL).
The electroluminescence display device is categorized in the inorganic light emitting diode display device and the organic light emitting diode display device according to the luminescence material. As a self-emitting display device, the electroluminescence display device has a response speed that is very fast, brightness that is very high and a large viewing angle.
FIG. 1 is a diagram illustrating the structure of the organic light emitting diode. As shown in FIG. 1, the organic light emitting diode comprises the organic light emitting material layer, and the cathode and the anode which are facing each other with the organic light emitting material layer therebetween. The organic light emitting material layer comprises the hole injection layer HIL, the hole transport layer HTL, the emission layer EML, the electron transport layer ETL and the electron injection layer EIL. The organic light emitting diode radiates the lights due to the energy from the excition formed at the excitation state in which the hole and the electron are recombined at the emission layer EML.
The organic light emitting diode radiates light due to the energy from the excition formed at the excitation state in which the hole from the anode and the electron from the cathode are recombined at the emission layer EML. The organic light emitting diode display can represent the video data by controlling the amount (or ‘brightness’) of the light generated and radiated from the emission layer ELM of the organic light emitting diode as shown in FIG. 1.
The organic light emitting diode display (or OLED) using the organic light emitting diode can be categorized in the passive matrix type organic light emitting diode display (or PMOLED) and the active matrix type organic light emitting diode display (or AMOLED).
The active matrix type organic light emitting diode display (or AMOLED) shows video data by controlling the current applied to the organic light emitting diode using the thin film transistor (or TFT).
FIG. 2 is the exemplary circuit diagram illustrating the structure of one pixel in the active matrix organic light emitting diode display (or AMOLED). FIG. 3 is a plane view illustrating the structure of one pixel in the AMOLED. FIG. 4 is a cross sectional view along the cutting line I-I′ for illustrating the structure of the AMOLED.
Referring to FIGS. 2 and 3, the active matrix organic light emitting diode display of the bottom emission type comprises a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light emitting diode OLED connected to the driving thin film transistor DT. By a scan line SL, a data line DL and a driving current line VDD disposed on a substrate SUB, a pixel area is defined. The organic light emitting diode OLED is formed in one pixel area so that it defines a light emitting area within the pixel area.
The switching thin film transistor ST is formed where the scan line SL and the data line DL cross. The switching thin film transistor ST selects the pixel which is connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the gate line GL, a semiconductor channel layer SA overlapping with the gate electrode SG, a source electrode SS and a drain electrode SD. The driving thin film transistor DT drives an anode electrode ANO of the organic light emitting diode OLED disposed at the pixel selected by the switching thin film transistor ST. The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light emitting diode OLED.
Referring to FIG. 4 more detail, on the substrate SUB of the bottom emission type active matrix organic light emitting diode display, the gate electrodes SG and DG of the switching thin film transistor ST and the driving thin film transistor DT, respectively are formed. On the gate electrodes SG and DG, the gate insulator GI is deposited. On the gate insulator GI overlapping with the gate electrodes SG and DG, the semiconductor layers SA and DA are formed, respectively. On the semiconductor layer SA and DA, the source electrode SS and DS and the drain electrode SD and DD facing and separating from each other are formed. The drain electrode SD of the switching thin film transistor ST is connected to the gate electrode DG of the driving thin film transistor DT via the contact hole penetrating the gate insulator GI.
The passivation layer PAS is deposited over the substrate SUB having the switching thin film transistor ST and the driving thin film transistor DT. A color filter CF may be formed at the anode electrode ANO area which is formed later. It is proper to form the color filter CF as it may have larger area as possible. For example, in the pixel area surrounded by the data line DL, the driving current line VDD and the former scan line SL, it is proper that the color filter CF may have the maximized area.
The upper surface of the substrate having the color filter CF is not even and/or smooth, but may be uneven and/or rugged having many steps. In order for the organic light emitting diode display to have good luminescent quality over the whole display area, the organic emission layer WOLE should be formed on an even or smooth surface. So, to make the upper surface in planar and even conditions, the over coat layer OC is deposited on the whole surface of the substrate SUB.
Then, on the over coat layer OC, the anode electrode ANO of the organic light emitting diode OLED is formed. Here, the anode electrode ANO is connected to the drain electrode DD of the driving thin film transistor DT through the contact hole penetrating the over coat layer OC and the passivation layer PAS.
In the interim, during one picture frame period, the data voltage Vdata should be applied to the anode electrode ANO with sufficient voltage amount for representing correct color value. To do so, a storage capacitance STG having enough capacitance for charging the anode electrode ANO within one picture frame period may be further included in the pixel area. As shown in FIG. 2, the storage capacitance STG can be formed between the drain electrode SD of the switching thin film transistor ST and the drain electrode DD of the driving thin film transistor DT. In detail, as shown in FIGS. 3 and 4, the storage capacitance may be formed by overlapping the gate electrode DG of the driving thin film transistor DT with the anode electrode ANO. In order to ensure the storage capacitance STG having enough capacity, it is required to ensure the overlapping area of the gate electrode DG of the driving thin film transistor DT with the anode electrode ANO as large as possible.
On the substrate SUB having the anode electrode ANO, a bank BANK is formed to distinguish the non-emission area having the switching thin film transistor ST, the driving thin film transistor DT and the various lines DL, SL and VDD from the emitting area having the organic light emitting diode OLED, for defining the light emitting area. The bank BANK can define the overlapping area of the organic emission layer OLE with the anode electrode ANO. Therefore, the light emitting area can be defined by the bank BANK. Generally, the light emitting area is included within the pixel area and has the maximum area as possible.
As explained above, the light emitting area is directly related to the color filter CF. Therefore, it is proper that the area of the color filter CF has a little larger area than the light emitting area defined by the bank BANK. That is, even though the color filter CF is formed as having maximized area, the actual light emitting area can be defined by the bank BANK.
The anode electrode ANO is exposed by the bank BANK. On the anode electrode ANO and the bank BANK, the organic emission layer WOLE is deposited. On the organic emission layer WOLE, the cathode electrode CAT is deposited sequentially. Consequently, the organic light emitting diode OLED connected to the driving thin film transistor DT is completed.
The enlarged diagram shown in the dotted circles of FIG. 3 illustrates the detailed structure of the light emitting area defined by the bank BANK. The anode electrode ANO may be formed as not to overlap with the data line DL, the former scan line SL and the driving current line VDD. As the organic light emitting diode is driven by current driving method, the anode electrode ANO is supplied with a large amount of the electric current via the driving current line VDD. Therefore, when the anode electrode ANO is overlapped with the lines DL, SL and/or VDD surrounding the anode electrode ANO, it is very highly possible that the parasitic capacitance is formed at the passivation layer PAS inserted therebetween.
Therefore, as shown in FIGS. 3 and 4, when forming the anode electrode ANO, it should have a first predetermined distance G1 from the driving current line VDD by considering the alignment margin in the patterning process. Furthermore, when forming the bank BANK on the anode electrode ANO, a second predetermined distance G2 inward the pixel area from the anode electrode ANO by considering the alignment margin in the patterning process. Consequently, as the bank BANK should be formed as covering some area of the anode electrode ANO, the rectangular area illustrated by alternated long and short dash line and hatched with inclined line may be the light emitting area. Further, as the organic emission layer WOLE is deposited on the anode electrode ANO, this light emitting area can define the aperture ratio of the pixel area. The light emitting area may have the loss amount corresponding to summation of the first and the second predetermined distances, G1+G2. Like this, at the data line DL side, the light emitting area may have the loss amount corresponding to summation of the first and the second predetermined distances, G1+G2. Generally, the alignment margin can be set as 1˜3 um (micrometer). Therefore, the total loss distance of the light emitting area may be 2˜6 um by the alignment margin of the manufacturing process.
Like this, in one pixel area, various elements including the thin film transistors ST and DT and the storage capacitance STG can be formed. In convenience, FIG. 3 illustrates two thin film transistors ST and DT. However, more thin film transistors can be further included to compensate the degraded characteristics of the elements due to various causes between elements. Due to this complexity, the aperture ratio, the ratio of the light emitting area to the pixel area, in the organic light emitting diode display according to the related art may be 30%.
In the organic light emitting diode display as mentioned above, the organic light emitting diode including the anode electrode ANO, the organic emission layer WOLE and the cathode electrode CAT naturally have its own capacitance. This is called ‘the anode capacitance’. Referring to FIG. 2 again, the anode capacitance Coled is represented as an equivalent circuit diagram of a capacitance formed between the two electrodes of the organic light emitting diode OLED.
As the anode capacitance Coled is increased, the luminescence (or brightness) of the organic light emitting diode can be increased. That is, in order to have higher or brighter luminescence with the same light emitting area, the anode capacitance Coled should have larger capacitance for better image quality. Therefore, when selecting the organic light emitting material, it is proper to select the organic material having higher permittivity. However, as the permittivity is the natural value of the material, it is very hard and restrictive to select organic light emitting material having enough permittivity we want.
Referring to FIG. 2, when the switching thin film transistor ST turns on as the scan voltage Vscan is supplied to the scan line SL, the data voltage Vdata supplied to the data line DL turns on the driving thin film transistor DT. Then, the electric current and voltage for driving the organic light emitting diode OLED is supplied to the organic light emitting diode OLED. At the same time, the data voltage Vdata is stored at the storage capacitance STG. At this time, the voltage supplied to the anode electrode ANO can be changed (or reduced) by the data voltage Vdata corresponding to the ratio of the storage capacitance Cst and the anode capacitance Coled. This change may cause the reduction of the luminescence (or brightness) of the organic emission layer OLE.ΔVanode=Vdata(Cst/Coled)  [Equation 1]
Here, Vanode means the anode voltage supplied to the anode electrode ANO, Cst means the capacity amount of the storage capacitance STG, and the Coled means the anode capacitance of the organic light emitting diode OLED.
According to the Equation 1, in order to minimize the change of the anode electrode Vanode, the capacitance value of Coled may be increased or the amount of the storage capacitance Cst may be reduced. As mentioned above, as the anode capacitance Coled is related to the natural value of the organic emission layer OLE, the most possible method is to reduce the capacity amount Cst of storage capacitance STG. However, the storage capacitance STG is for supplying certain voltage to the anode electrode ANO during one picture frame so that it should have enough capacitance for maintaining high capacity value. Therefore, the storage capacitance cannot be reduced freely as wanted.
By these various restrictions, the currently used maximized aperture ratio according to the related art is 30% at most. One possible method for getting the organic light emitting diode display having higher luminescence is to increase the electric current for driving the organic light emitting diode OLED. However, this method has another problem in that power consumption is increased. It is very hard to design and manufacture an organic light emitting diode display with satisfying the various conditions at the same time, such as having lowered parasitic capacitance, ensuring enough storage capacitance, and having higher aperture ratio.